1. Field of the Invention
The present invention is directed generally to steel rule dies for punching out sheet material such as folding carton blanks and, more particularly, to such dies of the type incorporating lightweight rigid plastic dieboard constructions.
2. Description of the Prior Art
Steel rule dies have been used for many years for cutting, creasing and perforating cardboard, paperboard and other sheet materials in making folded carton blanks. The steel rules are arranged in a predetermined pattern to form the desired creasing and cutting patterns in the carton blank. The steel rules are retained either between blocks held in a steel frame or chase by wedges or quoins (block dies) or within slots formed in a rigid die board (jig dies) which, as with the block dies, may be held in a chase with quoins. The male die retaining the steel rules cooperates with the female die, also called the counterplate, to make the impressions in the carton blank.
Depending on the type of construction, the dies have varying useful lives, the more durable of which may be required to make hundreds of thousands, or even millions, of impressions. It is very important to maintain the dimensional stability of the dies over long periods of time and throughout each production run. Moreover, the cutting and creasing edges of the rules wear out, loosen or otherwise become defective over time, and it is desirable to replace them by reknifing the dies rather than by having to replace the entire die. Then too, it is important to be able to efficiently reknife the dies with a minimum of cost.
The base material for the steel rule dies have been fabricated from a variety of materials including wood, laminated wood, metal, and plastic materials. Each offers various advantages and disadvantages, such as expense, weight, dimensional stability, wearability, etc. For example, a die base made of steel offers very high dimensional stability and durability but is relatively much more expensive to produce. Consequently, steel is the construction of choice for dies, especially counterplates, intended to be used over long periods of time and prolonged production runs. However, depending on its overall dimensions the die, and particularly the male die base containing the rules, may weigh up to several hundred pounds. Because of the excessive weight, such dies cannot be manually lifted and handled by one or even two men. As a result, labor costs to change dies becomes a significant and sometimes even prohibitive factor.
Because the steel counterplate is much thinner than the die board which receives the rules, the use of steel in the counterplate does not add significantly to overall weight. Since the use of one piece steel counterplates became common in the folding carton industry in approximately the 1970s, manufacturers of steel rule dies and die base materials have been searching for ways to maintain the relationship of die base to counterplate registration. It is vital that the two stay in close registration over large areas, for when one expands or shrinks at a different rate than the other, undesirable product is produced.
The need for dimensionally stable, low cost, lightweight die boards to accept the cutting and creasing rules is also due to the advances in the technology for cutting the rule slots. Die boards made of wood materials for example, while cheaper and lighter than steel, are also much less dimensionally stable due to their susceptibility to expansion, shrinkage and warpage. Traditionally, slots in wooden die boards have been made using a jig saw, and accuracy of placement of the slot locations depended heavily upon the skill of the human operator. In more recent times the use of lasers to create the slots for the rules has significantly increased this accuracy. To compare, a die board having slots made using a jig saw will typically allow dimensional accuracy of + or -0.015 inches over the entire die, whereas the slots in a laser cut die board are typically dimensionally accurate to + or -0.002 inches. Nevertheless, the tighter tolerances achieved by laser cutting techniques are lost over time in wooden die boards because of their susceptiblity to expansion, shrinkage and warpage.
With the increased dimensional accuracy offered by laser cutting techniques, attempts have been made to find suitable low cost, lightweight materials that offer greater inherent dimensional stability than wood materials such as maple, birch or plywoods.
Many different attempts have been made to stabilize cellulose fiber type products and other laser processable type materials for the end purpose of registering and maintaining the registration long term to the counterplate. Included among these was the use of sealant "dunk tanks" and the incorporation of resins in the die base material manufacturing process. One such material, known in the art as "Permaplex", is used to make a type of die called a "layered die." Layered dies includes a Permaplex inner core surrounded on the perimeter by steel rails and also by steel sheets on the top and bottom to fully encapsulate the core. The layered dies are characterized as being very heavy, more stable, and machine and labor intensive on assembly.
Another such effort which has found use is the bonded die, U.S. Pat. No. 3,863,550. This die includes two outer plates with an epoxy material in between that is poured in after the steel rules are installed in place. This die has high dimensional stability but is characterized by longer delivery times, being very heavy in weight, labor intensive and expensive to produce, and still having delamination tendencies.
U.S. Pat. No. 5,143,768 to Wilderman et al. teaches the use of a laminated die board structure comprising a rigid core of a plastic material, such as polyurethane, having a polyurea-cellulose composite secured thereto. While this structure offers the possibility of greater dimensional stability due to its greater ability to withstand the effects of temperature and humidity, dimensional stability is still compromised during the knifing process and also when the die board is positioned in the die chase. When for example slots (kerfs) are cut into the polyurethane material either by laser or jig saw, the width of the; slots is sized slightly less than the width of the rule to allow for a friction fit. As the rule is inserted into the slot, the pressure exerted against the sides of the slot by the rule expands the slot thereby changing the relative spacing and positioning of the rules. Also, when the die board is positioned in the die chase and pressure is applied by the quoins to hold the die board into place, too much pressure can be applied thereby causing undesired movement in the positioning of the rules.